There has heretofore been known thermoelectric materials of the type in which a heterojunction interface between a conductive layer and a barrier layer is formed into a flat atomic surface (see, for example, U.S. Pat. No. 5,436,467).
In the thermoelectric material, the energy gap in the barrier layer is maintained to be much wider than the energy gap in the conductive layer to create a large difference between the two energy gaps, whereby quantum wells are formed in the conductive layers. As a result, the electric conductivity of the thermoelectric material is heightened, and an improved thermoelectric performance is exhibited.
In order to strictly control the interfaces of the barrier layer and the conductive layer, therefore, the layers have heretofore been formed by the molecular beam epitaxial method (MBE), atomic epitaxial layer method (ALE) or the like method.
In a thermoelectric material of this type, however, there is a difference in the coefficient of thermal expansion between the barrier layer and the conductive layer, due to the difference in the crystalline structures of the semiconductors and the difference in the lattice constants. Therefore, when the heterojunction interface between the barrier layer and the conductive layer is formed into a flat atomic surface and then the temperature is elevated in order to maximize the thermoelectric performance of the thermoelectric material to its limit, a relatively large thermal stress is produced in the heterojunction interface. As a result, cracks are produced in the conductive layer and/or in the barrier layer due to the thermal stress, thereby breaking the thermoelectric material.
Furthermore, in order to form layers via the above-mentioned methods, very expensive equipment and very complex process control are required. In addition, despite the process control, the occurrence of defective products is high, resulting in an increase in the cost at which the thermoelectric materials are produced.
A need therefore exists for a thermoelectric material which has an excellent thermoelectric performance even when it is used at elevated temperatures, and which increases the productivity and lowers the cost of production. The present invention fulfills these needs, and provides further related advantages.